The present invention relates to identification tag readers and, in particular, to an antenna array having a rotating field around a vertical axis perpendicularly aligned to the travel path of an ID tag passing through the field to assure data capture.
A problem associated with RFID technology is that with any reader-tag combination various orientations of a tag relative to a reader antenna exist in which there is no communication between the tag and the reader. The missing data and ensuing errors are not readily compensated without performing redundant read operations. Inanimate tagged objects can be positioned to avoid the problem. However, tagged people, animals or objects having random orientations can pass through a reader magnetic field without being identified where the pattern of the reader field and the orientation of the tag provide marginal field coupling.
An illustrative example of the latter circumstance might occur with a cow passing through a reader station located in an “alleyway” or chute. The antennas used with this type of reader often consist of two vertical panels mounted parallel to each other about 3 feet apart. Each panel contains a large air coil. The coils are driven such that the individual fields are either opposing or enhancing. In either case, a cow can pass through the magnetic field established by the antennas with its tag oriented such that it cannot be detected.
U.S. Pat. No. 6,307,468 teaches the use of antenna coils mounted in such a way that the antenna coils do not exhibit a mutual inductance (i.e. a coupling of their respective fields). In particular, the antenna coils are driven with currents differing in phase by 90°. Fewer non-functional reader-tag orientations are thereby obtained.
A common construction of the foregoing antenna is to vertically mount one coil inside another with the planes of the coils positioned 90° apart. Such a configuration and drive results in a magnetic field inside the coil structure that appears to rotate within the coils, with the axis of rotation aligned to the intersection of the two coil planes. A tag passing through the field in any orientation except horizontal (i.e. perpendicular to the axis of rotation) can be read. In the case of ear tags on cows, the horizontal tag orientation is a very unlikely orientation.
A disadvantage of the foregoing crossed coil reader antenna is that the physical antenna coils include a wireway or conduit that mounts over the top and across the bottom of the walkway containing the coils, thus presenting an open-ended tubular alleyway. The conduit containing the antenna coils occupies four sides of a six sided cube space, which makes it difficult or impractical to implement and use. It can cause animals to balk at passing the antenna, which can interfere with the handling of the animals, particularly by riders on horseback.
The subject antenna and reader was therefore constructed to provide an antenna array with a rotating field between the coils of two antennas with no mutual inductance apparent between the coils. The elimination of mutual inductance is difficult to obtain in practice. The subject antenna constructions, however, provide coils that can be mounted in non-ideal arrangements with the mutual inductance between them being compensated or nulled by an external means. More convenient physical arrangements of the antennas are thereby made possible, including arrangements that allow open top alleyways.